The instant application should be granted the priority dates of Oct. 18, 2005, the filing date of the corresponding German patent application 10 2004 030 063 1, as well as Mar. 8, 2005, the filing date of the International patent application PCT/EP2005/002441.
The present invention relates to a method and a device for grinding ceramic spheres.
The term “ceramic spheres” is to be understood in the context of the present patent application as referring to spheres made of ceramic materials such as, for example, oxide ceramics, carbides, silicon nitride, precious and semiprecious stones but also glass.
Currently, the grinding of ceramic spheres for achieving low degrees of surface roughness and high quality classes is generally carried out using devices such as are also used for the machining of metal spheres. The ceramic spheres are in this case not actually ground but rather lapped. Whereas in the machining of metallic spheres there is provided initially coarse grinding and then fine grinding using grinding wheels with bound abrasive grains and lapping is optionally practiced thereafter with abrasive grains present in paste form, ceramic spheres are not machined using grinding wheels but rather lapped over the entire abrasion process. The abrasive grains present in the grinding paste are in this case generally in diamond form.
Technologically, this process s exceptionally difficult to carry out, for the abrasion rate is in the order of magnitude of at most 100 um per day. The abrasion to be realized of from 0.2 to 0.4 in sphere diameter corresponds to the thickness of the inhomogeneous boundary layer and is in some cases achieved only in a machining time of several days. In addition, after the lapping process, the ceramic spheres are markedly soiled by adhering grinding paste. In the conventional methods for washing the spheres, this grinding paste is in some cases very difficult to remove. The degree of wear of the two metal disc is extremely high during tapping with loose diamond grains. Finally, the very high consumption of diamonds greatly increases the costs of the method as a whole. As a result, the use of ceramic spheres has become established, especially in the field of ball bearings, only in applications in which costs are of secondary importance.
An attempt to improve the cost-effectiveness is found in U.S. Pat. No. 6,171,179 B1. In the grinder provided in said document, a grinding wheel is provided with electrolytically bound abrasive grains. The fixed guide disc has a number of guide rings which are each hydraulically loaded individually to ensure optimally uniform pressing of the ceramic spheres against the grinding wheel. This device has not proven successful in practice. It is believed that the service life of the grinding wheel is too short.
Japanese patent application JP 05042467 A discloses a method for the polishing of silicon nitride spheres using polishing discs having abrasive grains of from 5 to 60 percent by volume of Cr2Os with an average particle diameter of from 0.01 to 3 um. The machining of the spheres is very low with regard to the speed of abrasion of the surface. In a test, abrasion of 60 um was achieved over 50 hours, i.e. approximately 1 um per hour. The degree of surface roughness achieved in a second test is Ra=0.005 um. This method, which also proposes replacing a portion of the Cr2O3 with diamond, is suitable for achieving high surface qualities, although the abrasion rate is still unsatisfactory for the grinding of ceramic spheres.
The object of the present invention is therefore to provide a method and a device for grinding ceramic spheres allowing more economical manufacture of ceramic spheres having the requisite qualify and low divergence in the diameter of the spheres.